Hammermills are used in applications to reduce a wide range of materials including junked automobiles, friable materials such a limestone and coal and fibrous materials such as wood. Reducing or shredding these different types of material has resulted in a wide variety of designs and construction techniques to optimize the respective hammermill's performance in reducing or shredding the selected material. Automobile hammermills typically comprise apparatus that rotates relative to a cutter bar at the end of a feeder chute through which a junked automobile is fed. The rotating apparatus conventionally comprises four or six rows of hammers with each row including a plurality of hammers mounted for swinging movement on a common shaft or rotor which typically rotates at 700-900 revolutions per minute (RPM). For each revolution, each of the four or six rows of hammers passes by the cutter bar, shearing or shredding a junked automobile as it is fed along the entry chute. Due to the large size of an automobile hammermill, the tip speed of each hammer is very high.
Hammermills associated with reducing and/or shredding wood and bark are sometimes referred to as "wood or bark hogs." Hammer tip speeds are typically 11,000-15,000 feet per minute (FPM) for such wood and bark hogs. Many hammermill applications with swinging hammers have a tip speed of 11,000-12,000 FPM. This relatively high tip speed is required to have enough impact energy to properly reduce the material and to have enough force to hold the associated swinging hammers radially outward. Slow speed hammermills typically include a plurality of hammers which are rigidly mounted on a rotating shaft or rotor.
Some applications such as paper manufacturing require a large quantity of generally uniform sized wood chips. Oversized chips are typically screened out and resized or rechipped to the desired dimensions. Knife type rechippers with either disk or drum style rotors are often used to reduce oversized wood chips. Such knife type rechippers have a tendency to become plugged and frequently require substantial amounts of time and money for repair and maintenance. Hammermills that have previously been used to resize wood chips typically operate at tip speeds of around 10,000 FMP. The higher speeds produce a higher percentage of fines and/or undersized wood chips. Knife type rechippers also typically produce a substantial quantity of fines and oversized wood chips. The cost of resharpening and/or replacing knives associated with such rechippers can be as much as $20-30,000 per year. Knife type rechippers are also susceptible to damage from any metal trapped or contained in the oversized wood chips.
During the cooking process associated with manufacturing paper, fines and undersized chips typically produce a mush type mixture which does not contain cellulose fibers of the required length for high quality paper production. Both oversized and undersized chips are often not completely digested during the cooking process and result in waste material.